Nonethless we are building and operating complex systems.

Testing something 100% has diminishing returns.

Therefore -> you will never be able to prevent all issues. As you are not aware how many changes are happening, you don't have a value which could indicate the healthiness of it.

I personally think that when i'm getting older and better in my job, i still make errors, the amount and severity is going down though.

I'm sure they have both but complex shit breaks when you change it often.

Your devops team assures software quality?

But well, no QA effort is complete enough to really assure the quality of a large system. Thus, the GP holds true.